von Willebrand Disease (VWD) Testing Interpretations
VWF:Ag assay is a quantitative immunologic assay indicating the amount of VWF protein present in plasma.
The VWF:GPIbM Assay measures von Willebrand platelet-binding activity. The VWF:GPIbM, uses recombinant GPIba fragments with 2 gain-of-function variants to induce spontaneous binding of VWF without ristocetin. Although VWF:GPIbM eliminates the non-physiologic ristocetin, it does introduce a non-physiologic mutant GPIba receptor. VWF:GPIbM is reported to have an excellent coefficient of variation, a more sensitive lower limit of detection, and excellent correlation with VWF:RCo. The VWF GPIbM assay does not report falsely low values in individuals who possess the common p.D1472H polymorphism, and may report higher levels than the VWF ristocetin cofactor assay in patients with type 2B VWD.
VWF subunits assemble end-to-end into a series of different length multimers. High molecular weight multimers are more effective at both platelets and collagen binding compared with the intermediate and lower molecular weight multimers. Quantitative multimer analysis provides an objective measure of VWF multimer distribution to better characterize subtle changes observed in the subtypes of VWD and may help to determine the nature of any additional clinical laboratory testing to reach a clear-cut diagnosis.
The VWFpp level is used as a surrogate marker of VWF synthesis and secretion. VWFpp is most useful in identifying the subset of type 1 VWD subjects with very low VWF:Ag owing to increased clearance of VWF from plasma, which is commonly referred to as type 1C. These patients have a markedly reduced VWF half-life, which is as short as 1 to 3 hours after desmopressin administration, in contrast to the normal 8- to 12-hour half-life. Healthy individuals have a similar level of VWF and VWFpp in plasma, and they should have a steady-state ratio of <3.0. In type 1C patients, VWFpp is assumed to have a normal half-life, with substantially reduced VWF half-life resulting in an increased VWFpp/VWF:Ag ratio (>3.0). This ratio will also be elevated in patients with severe von Willebrand deficiency due to acquired VWD. VWFpp will be very low or absent in patients with type 3 VWD.
The Versiti Collagen III Binding assay detects the interaction of patient VWF with type III collagen. VWF collagen III binding is dependent on the presence of high-molecular weight VWF multimers, and patients with absence of high molecular weight VWF multimers (type 2A or type 2B) have reduced VWF:CB. Patients with normal multimer structure should have a VWF:CB III/VWF:Ag ratio of ~1.0. Patients with type 2A or type 2B VWD with abnormal VWF multimers have reduced VWF:CB III/VWF:Ag ratio. Assessment of VWF:CB III activity may have another role in identifying patients with collagen binding defects falling under the category of type 2M VWD who have a normal distribution of VWF multimers but abnormal CB III binding due to the rarer specific VWF A3 domain collagen binding variants.
The Versiti Collagen IV Binding Assay detects the interaction of patient VWF with type IV collagen and is designed to detect rare individuals with increased bleeding risk due to the A1 domain of VWF that binds collagens type IV and VI. Discrepancy between VWF:CBIV and VWF antigen in the absence of a multimer defect could indicate type 2M VWD due to a defect in the A1 domain.
This assay is designed to distinguish type 2N von Willebrand disease (an autosomal recessive disorder) from mild hemophilia A and hemophilia A carriers. Patient VWF is captured with monoclonal antibody in a microtiter well, any patient-derived FVIII:C is washed away and recombinant FVIII is then added. Patient-derived VWF antigen is quantitated immunologically, the bound FVIII is quantitated by chromogenic assay, and assay results are reported as the ratio of FVIII to VWF. Comparison is made to the results obtained with well-defined control plasmas (either compound type 1/type 2N VWD, 2N carrier state or normal). Numerical results and an interpretation are provided.
A ratio near 1.0 is observed for normal samples, vary low ratios are observed for samples obtained from patients with type 2N VWD. Intermediate ratios are observed for heterozygous carriers of type 2N VWD.
This assay panel is designed to identify type 2B von Willebrand disease. In the platelet-VWF binding assay, a monoclonal antibody to VWF is used to monitor the ability of patient plasma VWF to bind to formalin fixed platelets in the presence of low dose ristocetin. Comparison is made to the results obtained with well-defined control plasmas (either type 2B VWD or normal). Results are reported qualitatively as well as with an interpretive statement. Abnormal results are interpreted as indicating type 2B VWD, and normal are interpreted as indicating absence of type 2B VWD. Indeterminate results (with a recommendation for genetic confirmation) may be observed in rare 2B VWD patients in whom both high and intermediate molecular weight multimers are missing, and occasionally in patients with variants of the VWF gene that may increase in-vitro ristocetin-induced activation of VWF due to replacement of proline 1266 by either leucine or glutamine.